Piezoelectric element and piezoelectric element applied device

ABSTRACT

There is provided a piezoelectric element which includes a first electrode, a piezoelectric layer which is formed on the first electrode by using a solution method, and is formed from a compound oxide having a perovskite structure in which potassium, sodium, and niobium are provided, and a second electrode which is provided on the piezoelectric layer. A cross-sectional SEM image of the piezoelectric layer is captured at a magnification of 100,000. When evaluation is performed under a condition in which a measured value in a transverse direction is set to 1,273 nm, two or more voids are included in the piezoelectric layer, a difference between the maximum value and the minimum value among diameters of the voids to be largest in a film thickness direction is equal to or smaller than 14 nm, and the maximum value is equal to or smaller than 24 nm.

BACKGROUND

1. Technical Field

The present invention relates to a piezoelectric element which includesa first electrode, a piezoelectric layer, and a second electrode, and apiezoelectric element applied device which includes the piezoelectricelement.

2. Related Art

Generally, a piezoelectric element includes a piezoelectric layer andtwo electrodes. The piezoelectric layer has electromechanical conversioncharacteristics. The piezoelectric layer is interposed between the twoelectrodes. A device (piezoelectric element applied device) which usessuch a piezoelectric element as a driving source has been recentlyactively developed. As one of the piezoelectric element applied device,for example, a liquid ejecting head represented by an ink jet recordinghead, a MEMS element represented by a piezoelectric MEMS element, anultrasonic measurement device represented by an ultrasonic sensor andthe like, and a piezoelectric actuator device are provided.

As one of materials (piezoelectric materials) of a piezoelectric layerof a piezoelectric element, potassium sodium niobate (KNN; (K, Na)NbO₃)has been proposed (for example, see JP-A-2008-159807 and Japanese PatentNo. 4735840). In Japanese Patent No. 4735840, the piezoelectric layer isformed by using a solution method, and thus productivity is improved.

However, a KNN thin film which realizes at least one of improvement ofpiezoelectric characteristics and suppression of occurrence of a leakagecurrent is required.

If the KNN thin film is formed by using the solution method in whichhigh productivity is possible, there is a problem in that a void iseasily generated, and the void influences the piezoelectriccharacteristics and the occurrence of a leakage current. Because thepiezoelectric characteristics may be deteriorated by suppressing theoccurrence of the void, realization of improving the piezoelectriccharacteristics is not possible even by suppressing the occurrence ofthe void.

Such a problem is not limited to a piezoelectric element used in apiezoelectric actuator which is mounted in a liquid ejecting unitrepresented by an ink jet recording head, and similarly also occurs in apiezoelectric element used in other piezoelectric element applieddevices.

SUMMARY

An advantage of some aspects of the invention is to provide apiezoelectric element and a piezoelectric element applied device whichrealize at least one of improvement of piezoelectric characteristics andsuppression of the occurrence of a leakage current.

According to an aspect of the invention, there is provided apiezoelectric element which includes a first electrode, a piezoelectriclayer which is formed on the first electrode by using a solution method,and is formed from a compound oxide having a perovskite structure inwhich potassium, sodium, and niobium are provided, and a secondelectrode which is provided on the piezoelectric layer. Across-sectional SEM image of the piezoelectric layer is captured at amagnification of 100,000. When evaluation is performed under a conditionin which a measured value in a transverse direction is set to 1,273 nm,two or more voids are included in the piezoelectric layer, a differencebetween the maximum value and the minimum value among diameters of thevoids to be largest in a film thickness direction is equal to or smallerthan 14 nm, and the maximum value is equal to or smaller than 24 nm.

In the aspect, it is possible to suppress occurrence of a leakagecurrent by a void by setting the maximum value of the diameter of thevoid to be equal to or smaller than 24 nm. It is possible to suppresslocal generation of a leakage path and to suppress variation ofcharacteristics of the leakage current in the surface, by setting adifference between the maximum value and the minimum value amongdiameters of voids to be equal to or smaller than 14 nm. In apiezoelectric element using a direction for a piezoelectric constantd₃₁, it is possible to improve the piezoelectric characteristics bysetting the maximum value of the diameter of the void to be equal to orsmaller than 24 nm, and to suppress variation of the piezoelectriccharacteristics by setting the difference between the maximum value andthe minimum value among the diameters of voids to be equal to or smallerthan 14 nm.

Here, the number of the voids is preferably equal to or smaller than 8.In this manner, it is possible to improve the piezoelectriccharacteristics in a piezoelectric element using a direction for apiezoelectric constant d₃₁.

According to another aspect of the invention, there is provided apiezoelectric element applied device including the piezoelectricelement.

In the aspect, it is possible to provide a piezoelectric element applieddevice in which at least one of suppression of the occurrence of theleakage current, and improvement of the piezoelectric characteristics.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be described with reference to the accompanyingdrawings, wherein like numbers reference like elements.

FIG. 1 is a diagram illustrating a schematic configuration of arecording apparatus.

FIG. 2 is an exploded perspective view illustrating a recording head.

FIGS. 3A and 3B are a plan view and a cross-sectional view illustratingthe recording head.

FIG. 4 is an enlarged sectional view illustrating a piezoelectricelement.

FIGS. 5A to 5D are sectional views illustrating a manufacturing exampleof the recording head.

FIGS. 6A to 6C are sectional views illustrating the manufacturingexample of the recording head.

FIG. 7 is a diagram illustrating an I-V curve.

FIG. 8 is a diagram illustrating a P-V curve.

FIG. 9 is a diagram illustrating a P-V curve.

FIG. 10 is a diagram illustrating a P-V curve.

FIG. 11 is a diagram illustrating a P-V curve.

FIG. 12 is a diagram illustrating a P-V curve.

FIG. 13 is a diagram illustrating a P-V curve.

DESCRIPTION OF EXEMPLARY EMBODIMENTS

Hereinafter, an embodiment according to the invention will be describedwith reference to the drawings. The following descriptions are used fordescribing an aspect of the invention, and may be arbitrarily changed ina range of the invention. In the drawings, components denoted by thesame reference numerals indicate the same member as each other, anddescriptions thereof will be appropriately omitted. In FIGS. 2 to 6C, X,Y, and Z indicate three spatial axes perpendicular to each other. In thespecification, descriptions will be made by using directions along thethree spatial axes, which are respectively set as an X direction, a Ydirection, and a Z direction. The Z direction indicates a thicknessdirection or a layered direction of a plate, a layer, and a film. The Xdirection and the Y direction indicate an in-plane direction of theplate, the layer, and the film.

Embodiment 1

FIG. 1 illustrates an ink jet type recording apparatus which is anexample of a liquid ejecting apparatus according to an embodiment of theinvention. As illustrated in FIG. 1, in an ink jet type recordingapparatus I, an ink jet recording head unit (head unit) II whichincludes a plurality of ink jet recording heads is provided so as to beattachable to cartridges 2A and 2B. The cartridges 2A and 2B constitutean ink supply section. A carriage 3 having the head unit II mountedtherein is provided with a carriage shaft 5 so as to be movable in ashaft direction. The carriage shaft 5 is attached to a main body 4 ofthe apparatus. For example, the carriage 3 has a function of discharginga black ink composition and a color ink composition.

A driving force of a driving motor 6 is transferred to the carriage 3through a plurality of gears and timing belts 7 (not illustrate). Thus,the carriage 3 having the head unit II mounted therein is moved alongthe carriage shaft 5. A transporting roller 8 is provided as atransporting section in the main body 4 of the apparatus. A recordingsheet S which is a recording medium such as paper is transported by thetransporting roller 8. The transporting section that transports therecording sheet S is not limited to the transporting roller, and may bea belt, a drum, or the like.

According to such an ink jet type recording apparatus I, since the inkjet type recording apparatus I includes the ink jet recording head(simply also referred to the “recording head” below) according to theembodiment, as an ink jet recording head, it is possible to manufacturethe ink jet type recording apparatus I cheaply. Because improvement ofdisplacement characteristics of the piezoelectric element constituting apiezoelectric actuator is expected by using a piezoelectric elementwhich will be described in detail later, it is possible to improveejecting characteristics.

An example of a recording head 1 mounted in the above-described ink jettype recording apparatus I will be described with reference to FIGS. 2to 4. FIG. 2 is an exploded perspective view illustrating the recordinghead which is an example of a liquid ejecting unit according to theembodiment. FIG. 3A is a plan view of a piezoelectric element side of apassage formation substrate. FIG. 3B is a cross-sectional view takenalong line IIIB-IIIB in FIG. 3A. FIG. 4 is a sectional view obtained byenlarging a main portion of the piezoelectric element.

The passage formation substrate 10 (referred to as a substrate 10) isformed from, for example, a silicon single crystal substrate. In thepassage formation substrate 10, pressure generation chambers 12 areformed. In each of the pressure generation chambers 12 obtained bysubdivision with a plurality of partitions 11, a plurality of nozzleopenings 21 for discharging an ink of the same color are arranged in theX direction.

In the substrate 10, an ink supply path 13 and a communication path 14are formed on one end portion side of the pressure generation chamber 12in the Y direction. The ink supply path 13 is configured in such amanner that one side of the pressure generation chamber 12 is narrowedfrom the X direction and thus an opening area of the pressure generationchamber 12 becomes small. The communication path 14 has substantiallythe same width as the pressure generation chamber 12 in the X direction.A communication portion 15 is formed on the outside (+Y direction side)of the communication path 14. The communication portion 15 constitutes aportion of a manifold 100. The manifold 100 functions as a common inkchamber for the pressure generation chambers 12. In this manner, a fluidpassage which is formed from the pressure generation chamber 12, the inksupply path 13, the communication path 14, and the communication portion15 is formed in the substrate 10.

For example, a SUS nozzle plate 20 is bonded to one surface (surface ona −Z direction side) of the substrate 10. The nozzle openings 21 arearranged in the nozzle plate 20 in the X direction. The nozzle openings21 respectively communicate with the pressure generation chambers 12.The nozzle plate 20 may be bonded to the substrate 10 by using anadhesive, a heat-welding film, or the like.

A vibration plate 50 is formed on another surface (surface on a +Zdirection) of the substrate 10. The vibration plate 50 includes, forexample, an elastic film 51 formed on the substrate 10, and aninsulating film 52 formed on the elastic film 51. The elastic film 51 isformed from silicon dioxide (SiO₂), for example. The insulating film 52is formed from zirconium dioxide (ZrO₂), for example. The elastic film51 may not be a member separate from the substrate 10. A portion of thesubstrate 10 is processed so as to be thin, and a part obtained by theprocessing may be used as the elastic film.

A piezoelectric element 300 which includes a first electrode 60, apiezoelectric layer 70, and a second electrode 80 is formed on theinsulating film 52 with an adhesive layer 56 interposed between thepiezoelectric element 300. The adhesive layer 56 is used for improvingadhesiveness between the first electrode 60 and the substrate. Theadhesive layer 56 may be formed by using, for example, titanium oxide(TiO_(x)), titanium (Ti), silicon nitride (SiN), or the like. In a casewhere the adhesive layer 56 formed from titanium oxide (TiO_(x)),titanium (Ti), silicon nitride (SiN), or the like is provided, theadhesive layer 56 functions as a stopper when the piezoelectric layer 70(which will be described later) is formed, similar to the insulatingfilm 52. The stopper suppresses potassium and sodium which areconstituent elements of the piezoelectric layer 70, from beingtransmitted through the first electrode 60 and reaching the substrate10. The adhesive layer 56 may be omitted.

In the embodiment, displacement of the piezoelectric layer 70 havingelectromechanical conversion characteristics causes displacement tooccur in the vibration plate 50 and the first electrode 60. That is, inthe embodiment, the vibration plate 50 and the first electrode 60substantially have a function as a vibration plate. The elastic film 51and the insulating film 52 may be omitted and only the first electrode60 may function as the vibration plate. In a case where the firstelectrode 60 is directly provided on the substrate 10, the firstelectrode 60 is preferably protected by using an insulating protectivefilm and the like, so as not to bring an ink into contact with the firstelectrode 60.

The first electrode 60 is divided for each of the pressure generationchambers 12. That is, the first electrode 60 is configured as anindividual electrode which is independent for each of the pressuregeneration chambers 12. The first electrode 60 is formed so as to have awidth narrower than the width of each of the pressure generationchambers 12 in the X direction. The first electrode 60 is formed so asto have a width wider than the width of each of the pressure generationchambers 12 in the Y direction. That is, both end portions of the firstelectrode 60 are formed to the outside of an area facing the pressuregeneration chamber 12 in the Y direction. A lead electrode 90 isconnected to one end portion side (opposite side of the communicationpath 14) of the first electrode 60 in the Y direction.

The piezoelectric layer 70 is provided between the first electrode 60and the second electrode 80. The piezoelectric layer 70 is formed so asto have a width wider than the first electrode 60 in the X direction.The piezoelectric layer 70 in the Y direction is formed so as to have awidth wider than the length of the pressure generation chamber 12 in theY direction. An end portion (end portion on the +Y direction) of the inksupply path 13 side of the piezoelectric layer 70 in the Y direction isformed on the outside of an end portion of the first electrode 60. Thatis, another end portion (end portion on the +Y direction side) of thefirst electrode 60 is covered with the piezoelectric layer 70. One endportion (end portion on a −Y direction side) of the piezoelectric layeris provided on the inner side of one end portion (end portion on the −Ydirection side) of the first electrode 60. That is, the one end portion(end portion on the −Y direction side) of the first electrode 60 is notcovered with the piezoelectric layer 70.

The second electrode 80 is provided over the piezoelectric layer 70, thefirst electrode 60, and the vibration plate 50 in the X direction. Thatis, the second electrode 80 is configured as a common electrode which iscommonly used for a plurality of piezoelectric layers 70. Instead of thesecond electrode 80, the first electrode 60 may be used as the commonelectrode.

A protective substrate 30 is bonded to the substrate 10 in which thepiezoelectric element 300 is formed, by using an adhesive 35. Theprotective substrate includes a manifold portion 32. At least a portionof the manifold 100 is configured by the manifold portion 32. Themanifold portion 32 according to the embodiment is formed in a widthdirection (X direction) of the pressure generation chamber 12, so as topass through the protective substrate 30 in a thickness direction (Zdirection). As described above, the manifold portion 32 communicateswith the communication portion 15 of the substrate 10. With theconfiguration, the manifold 100 which functions as the common inkchamber for the pressure generation chambers 12 is configured.

A piezoelectric element holding portion 31 is formed in an areaincluding the piezoelectric element 300 in the protective substrate 30.The piezoelectric element holding portion 31 has a space which is largeenough not to impede the movement of the piezoelectric element 300. Thespace may be sealed or may not be sealed. A through-hole 33 which passesthrough the protective substrate 30 in the thickness direction (Zdirection) is provided in the protective substrate 30. An end portion ofthe lead electrode 90 is exposed in the through-hole 33.

A driving circuit 120 which functions as a signal processing unit isfixed to the protective substrate 30. The driving circuit 120 may use,for example, a circuit board or a semiconductor integrated circuit (IC).The driving circuit 120 and the lead electrode 90 are electricallyconnected to each other through a connection wire 121. The drivingcircuit 120 may be electrically connected to a printer controller 200.Such a driving circuit 120 functions as a control section according tothe embodiment.

A compliance substrate 40 which is formed from a sealing film 41 and afixation plate 42 is bonded to the protective substrate 30. An area ofthe fixation plate 42, which faces the manifold 100, functions as anopening portion 43 obtained by completely removing the fixation plate 42in the thickness direction (Z direction). One surface (surface on the +Zdirection side) of the manifold 100 is sealed only by the sealing film41 having flexibility.

Next, details of the piezoelectric element 300 will be described. Thepiezoelectric element 300 includes the first electrode 60, the secondelectrode 80, and the piezoelectric layer 70 provided between the firstelectrode 60 and the second electrode 80. The thickness of the firstelectrode 60 is about 50 nm. The piezoelectric layer 70 is a so-calledthin-film piezoelectric body. That is, the piezoelectric layer 70 has athickness of 50 nm to 2000 nm. The thickness of the second electrode 80is about 50 nm. The value of the thickness of the constituentsexemplified herein is only an example, and may be changed in a rangewithout deviating from the gist of the invention.

As the material of the first electrode 60 and the second electrode 80,precious metal such as platinum (Pt) and iridium (Ir) is appropriate.The material of the first electrode 60 and the material of the secondelectrode 80 may be materials having conductivity. The material of thefirst electrode 60 and the material of the second electrode 80 may bethe same as each other, or may be different from each other.

The piezoelectric layer 70 is formed by using a solution method, and isformed of compound oxide of compound oxide of a perovskite structure.The perovskite structure is indicated by a general formula ABO₃. Thecompound oxide contains potassium (K), sodium (Na), and niobium (Nb).That is, the piezoelectric layer 70 includes a piezoelectric materialformed from KNN-based compound oxide which is represented by thefollowing formula (1).

(K_(x),Na_(1-x))NbO₃  (1)

(0.1≦X≦0.9)

The compound oxide represented by the formula (1) is so-called aKNN-based compound oxide. The KNN-based compound oxide is anon-lead-based piezoelectric material in which the content of lead (Pb)and the like is suppressed. Thus, the KNN-based compound oxide hasexcellent biocompatibility, and has a small environmental load. Becausethe KNN-based compound oxide has excellent piezoelectric characteristicsamong non-lead-based piezoelectric materials, the KNN-based compoundoxide is advantageous for improving various types of characteristics. Inaddition, the KNN-based compound oxide has the Curie temperature higherthan that of other non-lead-based piezoelectric materials (for example,BNT-BKT-BT; [(Bi, Na)TiO₃]-[(Bi,K)TiO₃]-[BaTiO₃]), and occurrence ofdepolarization due to an increase of a temperature is also difficult.Thus, using at a high temperature is possible.

In the formula (1), the content of K is preferably from 30 mol % to 70mol % with respect to the total amount of metal elements constituting anA site (in other words, the content of Na is preferably from 30 mol % to70 mol % with respect to the total amount of the metal elementsconstituting the A site). That is, in the formula (1), a range of0.3≦x≦0.7 is preferable. According to this range, compound oxide havingadvantageous piezoelectric characteristics is obtained. The content of Kis preferably from 35 mol % to 55 mol % with respect to the total amountof the metal elements constituting the A site (in other words, thecontent of Na is preferably from 45 mol % to 65 mol % with respect tothe total amount of the metal elements constituting the A site). Thatis, a range of 0.35≦x≦0.55 is more preferable in the formula (1).According to this range, compound oxide having more advantageouspiezoelectric characteristics is obtained.

The piezoelectric material forming the piezoelectric layer 70 may be theKNN-based compound oxide, and is not limited to the compositerepresented by the formula (1). For example, another metal element(additive) may be included in an A site or a B site of potassium sodiumniobate. Examples of such an additive include manganese (Mn), lithium(Li), barium (Ba), calcium (Ca), strontium (Sr), zirconium (Zr),titanium (Ti), bismuth (Bi), tantalum (Ta), antimony (Sb), iron (Fe),cobalt (Co), silver (Ag), magnesium (Mg), zinc (Zn), and copper (Cu).

One or more types of this additive may be included. Generally, theamount of the additive is equal to or smaller than 20%, preferably equalto or smaller than 15%, and more preferably equal to or smaller than10%, with respect to the total amount of an element which functions asthe main component. Using the additive causes various types ofcharacteristics to be improved, and thus configurations or functions areeasily diversified. In a case where there is compound oxide containingthe above-described other elements, it is preferable that the ABO₃ typeperovskite structure is also provided.

Alkali metal in the A site may be excessively added to the composite ofthe stoichiometry. The alkali metal in the A site may be insufficientwith respect to the composite of the stoichiometry. Thus, the compoundoxide according to the embodiment may be represented by the followingformula (2). In the following formula (2), A indicates the amount of Kand Na which may be excessively added, or the amount of K and Na whichmay be insufficiently added. In a case where the amount of K and Na isexcessive, 1.0<A is satisfied. In a case where the amount of K and Na isinsufficient, A<1.0 is satisfied. For example, A being 1.1 means that Kand Na of 110 mol % are contained when the amount of K and Na in thecomposite of the stoichiometry is set to be 100 mol %. A being 0.9 meansthat K and Na of 90 mol % are contained when the amount of K and Na inthe composite of the stoichiometry is set to be 100 mol %. In a casewhere alkali metal of the A site is not excessively and notinsufficiently contained with respect to the composite of thestoichiometry, A is 1. From a viewpoint of improvement ofcharacteristics, 0.85≦A≦1.15 is satisfied, 0.90≦1.10 is preferablysatisfied, 0.95≦A≦1.05 is more preferably satisfied.

(K_(AX),Na_(A(1-X)))NbO₃  (2)

(0.1≦x≦0.9, preferably 0.3≦x≦0.7, and more preferably 0.35≦x≦0.55)

As the piezoelectric material, a material having a composite in whichsome of elements are absent, a material having a composite in which someof elements are surplus, and a material having a composite in which someof elements are substituted with other elements are also included. Amaterial shifted from a composite of stoichiometry by defect or surplus,or a material in which some of elements are substituted with otherelements are included in the piezoelectric material according to theembodiment, as long as the basic characteristics of the piezoelectriclayer 70 are not changed.

In the specification, “compound oxide of the ABO₃ type perovskitestructure containing K, Na, and Nb” is not limited only to the compoundoxide of the ABO₃ type perovskite structure containing K, Na, and Nb.That is, in the specification, the “compound oxide of the ABO₃ typeperovskite structure containing K, Na, and Nb” includes a piezoelectricmaterial which is represented as a mixed crystal which contains compoundoxide (for example, KNN-based compound oxide which is exemplified above)of the ABO₃ type perovskite structure containing K, Na, and Nb, andother compound oxide having the ABO₃ type perovskite structure.

In the scope of the invention, other compound oxide is not limited.However, as the other compound oxide, a non-lead-based piezoelectricmaterial which does not contain lead (Pb) is preferable. As the othercompound oxide, a non-lead-based piezoelectric material which does notcontain lead (Pb) and bismuth (Bi) is more preferable. If the compoundoxide is used, the piezoelectric element 300 having excellentbiocompatibility, and has a small environmental load is obtained.

The piezoelectric layer 70 formed from the above-described compoundoxide is preferentially oriented in a predetermined crystal plane in theembodiment. For example, the piezoelectric layer 70 formed fromKNN-based compound oxide is easily natural-oriented in (100) plane. Inaddition, the piezoelectric layer 70 may also be preferentially orientedin (110) plane or (111) plane by a predetermined orientation controllayer which is provided if necessary. The piezoelectric layer 70 whichis preferentially oriented in the predetermined crystal plane causesvarious types of characteristics to be improved easier than apiezoelectric layer which is randomly oriented. As the KNN-basedcompound oxide, compound oxide is preferable monoclinic and hasmaximized displacement because a polarization axis thereof is set to a caxis or a direction inclined from the c axis. In the specification,preferential orientation means that a crystal of which the content isequal to or greater than 50%, and preferably equal to or greater than80% is oriented in a predetermined crystal plane. For example, “beingpreferentially orientated in (100) plane” includes a case where allcrystals in the piezoelectric layer 70 are oriented in the (100) plane,and a case where crystals of the half or more (being equal to or greaterthan 50%, and preferably equal to or greater than 80%) are oriented inthe (100) plane.

Since the piezoelectric layer 70 is polycrystalline, stress in the planeis dispersed and becomes uniform. Thus, a layer in which occurrence ofbreaking of the piezoelectric element 300 due to the stress isdifficult, and reliability is improved is obtained.

When a cross-sectional SEM of the piezoelectric layer 70 is captured ata magnification of 100,000, and evaluation is performed under acondition that a measured value in the captured cross-sectional SEM in atransverse direction is set to 1273 nm, a difference between the maximumvalue and the minimum value of the diameters of voids in a certain Zdirection in the film thickness direction is equal to or smaller than 14nm, and the maximum value of the diameters of the voids is equal to orsmaller than 24 nm. According to this range, the maximum value of thediameters of voids which are formed in the piezoelectric layer 70 is setto be equal to or smaller than 24 nm, and thus it is possible tosuppress that the voids function as a leakage path and are broken by aleakage current. The difference between the maximum value and theminimum value of the diameters of the voids formed in the piezoelectriclayer 70 is set to be equal to or smaller than 14 nm, and thus it ispossible to suppress local generation of a leakage path, and to use thepiezoelectric layer 70 which has piezoelectric characteristics uniformin an in-plane direction including the X direction and the Y direction.The “diameter of the void” stated in the embodiment refers to a width wof the largest portion of each of the voids 75 included in thepiezoelectric layer 70, in the Z direction in a section parallel to theZ direction which is the film thickness direction of the piezoelectriclayer 70, as illustrated in FIG. 4.

The number of voids 75 included in the piezoelectric layer 70 ispreferably equal to or smaller than 8, in the evaluation. Here, in ageneral case of the piezoelectric element 300 using the piezoelectricconstant d₃₁, the voids 75 included in the piezoelectric layer 70 arethe reason of blocking displacement in a direction of the piezoelectricconstant d₃₁. Thus, no void 75 in the piezoelectric layer 70 causesimprovement of the displacement characteristics. On the contrary, in acase of the piezoelectric element 300 using a piezoelectric constantd₃₃, it is known that providing voids 75 in the piezoelectric layer 70causes the piezoelectric layer 70 to be easily moved, and allows thedisplacement characteristics to be improved. That is, it is possible toobtain a large displacement amount with the same driving voltage.Accordingly, in the case where the piezoelectric element 300 uses thepiezoelectric constant d₃₁, it is preferable that the voids 75 are notpresent in the piezoelectric layer 70 as much as possible, and in theevaluation, it is appropriate that the number of voids 75 is equal to orsmaller than 8. Thus, it is possible to improve the piezoelectriccharacteristics of the piezoelectric element 300. On the contrary, inthe case where the piezoelectric element 300 uses the piezoelectricconstant d₃₃, it is preferable that the voids 75 are present in thepiezoelectric layer 70. If diameters of the voids 75, particularly, themaximum value w in the Z direction is excessively large, the voidsfunction as the leakage path and the leakage current is increased. Thus,it is preferable that the maximum value of diameters of the voids 75 isas small as possible. Accordingly, even in the case where thepiezoelectric element 300 uses the piezoelectric constant d₃₃, when across-sectional SEM is captured at a magnification of 100,000 andevaluation is performed under a condition that a measured value in thecaptured cross-sectional SEM in the transverse direction is set to 1273nm, the maximum value of the diameters of the voids 75 is set to beequal to or smaller than 24 nm, and thus it is possible to suppress theoccurrence of the leakage current. In addition, the difference betweenthe maximum value and the minimum value of the diameters of the voids 75in the evaluation is set to be equal to or smaller than 14 nm, and thusit is possible to suppress formation of the local leakage path in thepiezoelectric layer 70 and it is possible to cause the characteristicsto be uniform in the in-plane direction of the piezoelectric layer 70,that is, in the direction including the X direction and the Y direction,by suppressing the occurrence of the leakage current. Further, thepiezoelectric elements 300 which uses the piezoelectric constant d₃₁ andd₃₃ in the embodiment respectively correspond to a piezoelectric elementusing a length extension and reduction (d₃₁) mode, and a piezoelectricelement using a width extension and reduction (d₃₃) mode. Thepiezoelectric element 300 according to the embodiment corresponds to thepiezoelectric element using the piezoelectric constant d₃₁. Thepiezoelectric element using the piezoelectric constant d₃₃ iswell-known, for example, as illustrated in FIG. 13 of JP-A-6-340075.Thus, detailed descriptions thereof will be omitted.

Next, an example of a manufacturing example of the recording headaccording to the embodiment will be described with reference to FIGS. 5Ato 6C. Firstly, the vibration plate 50 is formed on a wafer 110 which isa silicon wafer. In the embodiment, the vibration plate 50 is formedfrom silicon dioxide (elastic film 51) and zirconium oxide (insulatingfilm 52). The silicon dioxide is formed by thermally oxidizing the wafer110. The zirconium oxide is formed in such a manner that a film isformed by using a sputtering method, and then the formed film isthermally oxidized. Then, the adhesive layer 56 formed from titaniumoxide is formed on the vibration plate 50 by using a sputtering methodor by thermally oxidizing a titanium film. As illustrated in FIG. 5A,the first electrode 60 is formed on the adhesive layer 56 by using asputtering method, an evaporation method, or the like.

Then, as illustrated in FIG. 5B, a resist (not illustrated) having apredetermine shape is formed as a mask on the first electrode 60. Theadhesive layer 56 and the first electrode 60 are simultaneouslypatterned. Then, as illustrated in FIG. 5C, a plurality of piezoelectricfilms 74 are formed so as to be superposed on the adhesive layer 56, thefirst electrode 60, and the vibration plate 50. The piezoelectric layer70 is formed by the plurality of piezoelectric films 74. Thepiezoelectric layer 70 may be formed by using a solution method(chemical solution method) such as a MOD method and a sol-gel method,for example. The piezoelectric layer 70 is formed by using such asolution method, and thus it is possible to improve the productivity ofthe piezoelectric layer 70. The piezoelectric layer 70 formed by usingsuch a solution method is formed by repeating a series of processes aplurality of number of times. The series of processes includes processesfrom a process (coating process) of performing coating with a precursorsolution to a process (baking process) of baking the precursor film.

Specific procedures in a case where the piezoelectric layer 70 is formedby using the solution method are as follows, for example. Firstly, aprecursor solution containing a predetermined metal complex is prepared.In the precursor solution, a metal complex for forming compound oxidecontaining K, Na, and Nb by baking is dissolved or dispersed in anorganic solvent by baking. At this time, a metal complex containing anadditive such as Mn may be mixed.

Examples of the metal complex containing K include potassium2-ethylhexanoate, potassium carbonate, and potassium acetate. Examplesof the metal complex containing Na include sodium 2-ethylhexanoate,sodium carbonate, and sodium acetate. Examples of the metal complexcontaining Nb include 2-ethyl hexane acid niobium and pentaethoxyniobium. In a case where Mn is added as the additive, examples of themetal complex containing Mn include manganese 2-ethylhexanoate. At thistime, two or more types of metal complex may be used together. Forexample, as the metal complex containing K, potassium carbonate andpotassium acetate may be used together. As a solvent, 2-n-butoxyethanol,n-octane, a solvent mixture of 2-n-butoxyethanol and n-octane, and thelike are exemplified. The precursor solution may contain an additiveagent for stabilizing dispersion of the metal complex containing K, Na,and Nb. As such an additive agent, 2-ethyl hexane acid and the like areexemplified.

The coating with the precursor solution is performed on the wafer 110 onwhich the vibration plate 50, the adhesive layer 56, and the firstelectrode 60 are formed, thereby a precursor film is formed (coatingprocess). Then, the precursor film is heated to a predeterminedtemperature, for example, to a temperature of about 130° C. to 250° C.,and is dried for a predetermined period (drying process). Then, thedried precursor film is heated to a predetermined temperature, forexample, to a temperature of about 300° C. to 450° C., and is held for apredetermined period, and thereby being degreased (degreasing process).Finally, if the degreased precursor film is heated to a highertemperature, for example, to a temperature of 650° C. to 800° C., and isheld at this temperature, and thereby being crystallized, apiezoelectric film 74 is completed (baking process). It is appropriatethat a heating rate in the drying process is set to be 15° C./sec to450° C./sec. The piezoelectric film 74 is baked at such a heating rate,and thus although being described in detail later, it is possible toadjust the size and the number of the voids formed in the piezoelectricfilm 74. The “heating rate” stated herein defines a time change rate ofthe temperature when the temperature reaches a desired bakingtemperature from 350° C.

As a heating device used in the drying process, the degreasing process,and the baking process, for example, a rapid thermal annealing (RTA)device, a hot plate, and the like are exemplified. The RTA deviceperforms heating by irradiation with an infrared lamp. The aboveprocesses are repeated a plurality of number of times, and thus thepiezoelectric layer 70 formed from a plurality of piezoelectric films 74is formed. In the series of the processes from the coating process tothe baking process, the processes from the coating process to thedegreasing process may be repeated a plurality of number of times, andthen, the baking process may be performed.

Before or after the second electrode 80 is formed on the piezoelectriclayer 70, if necessary, re-heating treatment (post annealing) may beperformed in a temperature range of 600° C. to 800° C. It is possible toform a good interface between the piezoelectric layer 70, and the firstelectrode or the second electrode 80, and to improve crystallinity ofthe piezoelectric layer 70, by performing such post annealing.

In the embodiment, the piezoelectric material contains alkali metal (Kor Na). The alkali metal is easily diffused in the first electrode 60 orthe adhesive layer 56, in the baking process. If the alkali metalreaches the wafer 110 through the first electrode 60 and the adhesivelayer 56, the alkali metal is caused to react with the wafer 110.However, in the embodiment, the insulating film 52 formed from thezirconium oxide layer conducts the stopper function of K or Na. Thus, itis possible to suppress reaching of the alkali metal to the wafer 110which is a silicon substrate.

After that, the piezoelectric layer 70 formed from a plurality ofpiezoelectric films 74 is patterned so as to have a shape as illustratedin FIG. 5D. Patterning may be performed by using dry etching such asreactive ion etching and ion milling, or wet etching in which an etchingliquid is used. Then, the second electrode 80 is formed on thepiezoelectric layer 70. The second electrode 80 may be formed by using amethod similarly to the first electrode 60. With the above processes,the piezoelectric element 300 which includes the first electrode 60, thepiezoelectric layer 70, and the second electrode 80 is completed. Inother words, a portion at which the first electrode 60, thepiezoelectric layer, and the second electrode 80 overlap each otherfunctions as the piezoelectric element 300.

Then, as illustrated in FIG. 6A, a wafer 130 for the protectivesubstrate is bonded to a surface on the piezoelectric element 300 sideof the wafer 110, through the adhesive 35 (see FIG. 3B). Then, thesurface of the wafer 130 for the protective substrate is abraded so asto become thin. The manifold portion 32 or the through-hole 33 (see FIG.3B) is formed on the wafer 130 for the protective substrate. Then, asillustrated in FIG. 6B, a mask film 53 is formed on a surface of thewafer 110 on an opposite side of the piezoelectric element 300, and ispatterned so as to have a predetermined shape. As illustrated in FIG.6C, anisotropic etching (wet etching) with an alkaline solution such asKOH is performed on the wafer 110 through the mask film 53. Thus, theink supply path 13, the communication path 14, and the communicationportion 15 (see FIG. 3B) are formed in addition to the pressuregeneration chamber 12 corresponding to each piezoelectric element 300.

Then, an unnecessary portion of an outer circumferential portion of thewafer 110 and the wafer 130 for the protective substrate is cut out andremoved by dicing and the like. The nozzle plate 20 is bonded to thesurface of the wafer 110 on the opposite side of the piezoelectricelement 300 (see FIG. 3B). The compliance substrate 40 is bonded to thewafer 130 for the protective substrate (see FIG. 3B). With the processuntil here, an assembly of chips for the ink jet recording head 1 iscompleted. The assembly is divided for each of the chips, and thus, theink jet recording head 1 is obtained.

Examples

Examples of the invention will be described below.

Examples 1 to 3

A surface of a silicon substrate which is used as the substrate 10 wasthermally oxidized, and thus the elastic film 51 formed from silicondioxide was formed on the silicon substrate. Then, a zirconium film wasformed on the elastic film 51 by sputtering, and the zirconium film wasthermally oxidized. Thus, the insulating film 52 formed from zirconiumoxide was formed. Then, a titanium film was formed on the insulatingfilm 52 by sputtering, thereby forming the adhesive layer 56. After aplatinum film was formed on the adhesive layer 56 by using a sputteringmethod, the platinum film was patterned so as to have a predeterminedshape. Thus, the first electrode 60 having a thickness of 50 nm wasformed.

Then, the piezoelectric layer 70 was formed through the followingprocedures. Firstly, an n-octane solution of potassium acetate, ann-octane solution of sodium acetate, and an n-octane solution ofpentaethoxy niobium were mixed, thereby preparing a precursor solution.Three types of precursor solutions were prepared. The three types ofprecursor solutions were a precursor solution (Example 1) having acomposite in which in which the value of “x” is 0.3811 in the followingformula (3), a precursor solution (Example 2) having a composite inwhich the value of “x” is 0.412, and a precursor solution (Example 3)having a composite in which the value of “x” is 0.4429.

(K_(x)Na_(1-x))(Nb_(0.995)Mn_(0.005))O₃ (x=0.3811, 0.412, and0.4429)  (3)

Then, coating with the prepared precursor solution was performed on thesilicon substrate on which the first electrode 60 had been formed, byusing a spin coating method (coating process). Then, the siliconsubstrate was mounted on a hot plate, and was dried at 180° C. for fourminutes (drying process). Then, the silicon substrate on the hot platewas degreased at 380° C. for four minutes (degreasing process). Bakingwas performed at 700° C. for three minutes by a RTA device (bakingprocess). The heating rate in the baking process was set to 350° C./sec.The heating rate stated herein refers to a time change rate of thetemperature when the temperature reaches 700° C. from 350° C. Theprocesses from such a coating process to such a baking process wererepeated ten times, and thus the piezoelectric layer 70 which wasconfigured by 10 piezoelectric films 74 and had a thickness of being setto 700 nm was formed.

A platinum film was formed on the piezoelectric layer 70 by using asputtering method, and thereby the second electrode 80 having athickness of 100 nm was formed.

After that, re-heating (post annealing) was performed at 650° C. forthree minutes by using a RTA device, and thus piezoelectric elements ofExamples 1 to 3 were formed. 10 pieces of the piezoelectric element ineach of Examples 1 to 3 were formed.

Comparative Examples 1 to 3

Piezoelectric elements of Comparative Examples 1 to 3 were formed byusing a composite and processes which were similar to those in theabove-described Example 1, except that the heating rate in the bakingwas set to 23° C./sec. pieces of the piezoelectric element in each ofComparative Examples 1 to 3 were formed.

Evaluation Details Scanning Electron Microscope Observation

A cross-section of a piezoelectric layer in each of the piezoelectricelements in Examples 1 to 3 and Comparative Examples 1 to 3 was observedby using a scanning electron microscope (SEM). Specifically, thecross-section of the piezoelectric layer was captured at a magnificationof 100,000, and the number of voids and diameters of the voids weremeasured under a condition in which a measured value in a transversedirection is set to 1,273 nm. Results obtained by measuring are shown inthe following Table 1. The diameter of each of the voids is the diameterat the largest portion of each of the void in the Z direction, in across-section parallel to the Z direction which is the film thicknessdirection of the piezoelectric layer 70. A piezoelectric element wasselected from 10 pieces of the piezoelectric element in each of Examples1 to 3 and Comparative Examples 1 to 3, and a cross-sectional SEM of theselected piezoelectric element was captured.

TABLE 1 Diameter of void (nm) Compar- Compar- Compar- ative ative ativeExam- Exam- Exam- Exam- Exam- Exam- ple 1 ple 2 ple 3 ple 1 ple 2 ple 31 23.8 19.8 19.8 13.9 15.8 27.7 2 15.9 17.9 21.8 13.9 33.6 23.8 3 15.915.9 21.8 19.8 39.6 31.7 4 17.9 19.8 25.8 31.7 17.8 5 23.8 21.8 21.833.7 6 9.92 47.6 33.6 13.9 7 17.9 43.6 45.6 21.8 8 13.9 43.6 45.5 45.5 925.8 19.8 43.6 10 25.8 35.6 31.7 11 37.7 51.4 37.9 12 27.8 29.7 35.6 137.98 33.6 35.6 14 33.7 31.7 35.6 15 33.7 13.8 43.6 16 41.6 33.6 35.5 1727.8 39.6 45.5 18 17.8 25.7 19 49.6 20 57.5 Maxi- 23.8 19.8 21.8 57.551.4 45.5 mum value (nm) Mini- 9.92 15.9 19.8 7.98 13.8 13.9 mum value(nm) Differ- 13.9 3.9 2.0 49.5 37.6 31.6 ence (nm) Number 8 4 3 20 17 18of voids

As shown in Table 1, in Examples 1 to 3, the maximum value of thediameters of the voids was equal to or smaller than 23.8 nm, and thedifference between the maximum value and the minimum value of thediameters of the voids was equal to or smaller than 13.9 nm. On thecontrary, in Comparative Examples 1 to 3, the maximum value of thediameters of the voids was equal to or greater than 57.5 nm, and thedifference between the maximum value and the minimum value of thediameters of the voids was equal to or greater than 49.5 nm.

In Examples 1 to 3, the number of voids was equal to or smaller than 8.However, in Comparative Examples 1 to 3, many voids (the number of voidswas equal to or greater than 17) were formed.

In this manner, the maximum value and variation of the diameters of thevoids, or the number of voids may be adjusted by changing the heatingrate in the baking process. The maximum value and the variation of thediameters of the voids, or the number of voids are influenced by thecomposite of the solution which forms the piezoelectric layer 70, orheating conditions in other heating process. Examples of the heatingconditions include a temperature and a time period in the bakingprocess, a temperature, a time period, and a heating rate in the dryingdegreasing process, a temperature a time period, and a heating rate inthe re-heating treatment, and the like.

Here, as described above, even when the piezoelectric element 300 usingdisplacement in either direction of the piezoelectric constants d₃₁ andd₃₃ is provided, if the maximum value of diameters of voids isexcessively large, the voids function as the leakage path and theleakage current is increased. Thus, as in Examples 1 to 3, the maximumvalue of diameters of voids included in the piezoelectric layer 70 isset to be equal to or smaller than 24 nm, and thus, in Examples 1 to 3,it is possible to suppress an increase of the leakage current incomparison to Comparative Examples 1 to 3. In addition, the differencebetween the maximum value and the minimum value of the diameters of thevoids, that is, variation of the diameters of the voids being smallcauses occurrence of local leakage in the surface of the piezoelectriclayer 70 to be difficult. Accordingly, as in Examples 1 to 3, thedifference between the maximum value and the minimum value of thediameters of the voids is set to be equal to or smaller than 14 nm, andthus it is possible to cause the characteristics of the leakage currentin the in-plane direction of the piezoelectric layer 70, that is, thedirection including the X direction and the Y direction to becomeuniform in comparison to Comparative Examples 1 to 3.

In a case where the piezoelectric element uses the piezoelectricconstant d₃₁, the voids 75 are the reason of blocking displacement in adirection of the piezoelectric constant d₃₁. Thus, in the evaluation, ina case where the piezoelectric element 300 uses the direction of thepiezoelectric constant d₃₁, the piezoelectric elements in Examples 1 to3 can improve the piezoelectric characteristics, that is, obtain a largedisplacement amount with the same driving voltage in comparison toComparative Examples 1 to 3. On the contrary, in a case where thepiezoelectric element 300 uses the direction of the piezoelectricconstant d₃₃, the piezoelectric layer may be easily moved and thepiezoelectric characteristics may be improved, by the voids beingpresent. Accordingly, it is considered that the piezoelectriccharacteristics of the piezoelectric elements in Comparative Examples 1to 3 are higher than those of the piezoelectric elements in Examples 1to 3, in a case where the piezoelectric element 300 uses thepiezoelectric constant d₃₃. Since the maximum value of the diameters ofthe voids in each of the piezoelectric elements in Comparative Examples1 to 3 is large and the variation in diameter of the void is large, theleakage path is easily generated and the characteristics of the leakagecurrent in the surface of the piezoelectric layer vary. Accordingly,even when the piezoelectric element using the piezoelectric constant d₃₃is used, it is preferable that the piezoelectric elements in Examples 1to 3 are used.

I-V Characteristics

In the piezoelectric element in Example 2, and the piezoelectric elementin Comparative Example 2, a voltage of ±50 V was applied, and arelationship between a current (I) and the voltage (V) was evaluated.The current (I) and the voltage (V) were measured in the air by using“4140B” manufactured by Hewlett-Packard Company, and by setting aholding time in measuring to two seconds. Results obtained by measuringare illustrated in FIG. 7.

As illustrated in FIG. 7, it was recognized that the piezoelectricelement in Example 2 has a tendency of current density (leakage current)smaller than that in Comparative Example 2. That is, it was recognizedthat, according to Example 2, it is possible to reduce the leakagecurrent in comparison to Comparative Example 2.

P-V Characteristics

In the piezoelectric elements in Examples 1 to 3 and the piezoelectricelements in Comparative Examples 1 to 3, a triangular wave having afrequency of 1 kHz was applied at room temperature (25° C.) in “FCE 1A”(manufactured by Toyo Co) by using an electrode pattern having φ of 500μm, and a relationship between a polarization amount (P) and a voltage(V) was obtained. Hysteresis curves of the piezoelectric element inExamples 1 to 3 are respectively illustrated in FIGS. 8, 9, and 10.Hysteresis curves of the piezoelectric element in Comparative Examples 1to 3 are respectively illustrated in FIGS. 11, 12, and 13.

As illustrated in FIGS. 8 to 10, in the piezoelectric elements inExamples 1 to 3, a good hysteresis curve derived from ferroelectricproperties was observed even when measurement with 50 V was performed.On the contrary, as illustrated in FIGS. 11 to 13, in the piezoelectricelements in Comparative Examples 1 to 3, the hysteresis curve becomesround due to the leakage current at the voltage on a positive side. Inone of the pieces of the piezoelectric element in Comparative Example 1,the hysteresis curve in the evaluation with an application of 50 V wasbroken.

Other Embodiments

Hitherto, the embodiment of the piezoelectric material or thepiezoelectric element, and the liquid ejecting unit and the liquidejecting apparatus which have the piezoelectric element mounted therein,according to the invention is described. However, the basicconfiguration of the invention is not limited to the above-describedform. For example, in Embodiment 1, the silicon single crystal substrateas the substrate 10 is exemplified. However, it is not limited thereto,and may use, for example, a SOI substrate or a material such as glass.

In Embodiment 1, as an example of the liquid ejecting unit, the ink jetrecording head is exemplified and described. However, the invention maybe widely applied to all types of the liquid ejecting head, and may beapplied to a liquid ejecting unit which ejects a liquid other than anink. Examples of such a liquid ejecting head include various recordingheads used in an image recording apparatus such as a printer; a coloringmaterial ejecting head used in manufacturing a color filter in a liquidcrystal display and the like; an electrode material ejecting head usedin forming an electrode in an organic EL display, a field emissiondisplay (FED), and the like; and a bio-organic material ejecting headused in manufacturing a bio-chip.

The invention is not limited to the piezoelectric element mounted in theliquid ejecting head, and may be also applied to a piezoelectric elementmounted in other piezoelectric element applied devices. As an example ofthe piezoelectric element applied device, an ultrasonic device, a motor,a pressure sensor, a pyroelectric element, and a ferroelectric elementmay be exemplified. A finished article using the piezoelectric elementapplied device, for example, an ejecting apparatus of a liquid and thelike, which uses an ejecting head for the liquid and the like; anultrasonic sensor using the ultrasonic device; a robot using the motoras a driving source; an IR sensor using the pyroelectric element; aferroelectric memory using the ferroelectric element may be included asthe piezoelectric element applied device.

The thickness, the width, the relative positional relationship, and thelike of the constituents illustrated in the drawings, that is, thelayers and the like may be exaggeratedly illustrated in describing theinvention. The term of “being on” in the specification is not limited tothe meaning that the positional relationship between the constituents is“just on”. For example, an expression of “the first electrode on thesubstrate” or “the piezoelectric layer on the first electrode” includesa case where other constituents are provided between the substrate andthe first electrode or between the first electrode and the piezoelectriclayer.

The entire disclosure of Japanese Patent Application No. 2015-112680,filed Jun. 2, 2015 is expressly incorporated by reference herein.

What is claimed is:
 1. A piezoelectric element comprising: a firstelectrode; a piezoelectric layer which is formed on the first electrodeby using a solution method, and is formed from a compound oxide having aperovskite structure in which potassium, sodium, and niobium areprovided; and a second electrode which is provided on the piezoelectriclayer, wherein a cross-sectional SEM image of the piezoelectric layer iscaptured at a magnification of 100,000, and when evaluation is performedunder a condition in which a measured value in a transverse direction isset to 1,273 nm, two or more voids are included in the piezoelectriclayer, a difference between the maximum value and the minimum valueamong diameters of the voids to be largest in a film thickness directionis equal to or smaller than 14 nm, and the maximum value is equal to orsmaller than 24 nm.
 2. The piezoelectric element according to claim 1,wherein the number of voids is equal to or smaller than
 8. 3. Apiezoelectric element applied device comprising the piezoelectricelement according to claim
 1. 4. A piezoelectric element applied devicecomprising the piezoelectric element according to claim 2.